1. Field
Devices and methods consistent with exemplary embodiments relates to a data reception device that receives transmitted image packets transmitted from a transmission device in a data transmitting system, which transmits image data in a stream-type data, and a program used in the data transmitting system.
2. Description of the Related Art
When image data is transmitted via the Internet, a stream-type transmitting method is often used in contrast to a download-type transmitting method. The stream-type transmitting method is a method that may transmit the image data from a transmission device to a reception device that then reproduces the image data in a perceived simultaneous manner to a user.
The image data that has been coded using a scalable video coding (SVC) method may be used when the stream-type transmitting method is executed, to transmit the image data depending on the circuit status or ability of a receiving end.
An extension of the H.264 video coding standards is an example of the SVC method. As shown in FIG. 6, the data structure of the coding may be divided into a base layer, which is formed with the minimum moving image elements needed, and an extended layer which is combined with the base layer to thereby increase image quality.
The base layer is a highly-reliable channel because the base layer is an essential data stream for reproducing an image on the receiving end. The base layer is transmitted and received with high reliability using redundancy checks such as forward error correction (FEC).
On the other hand, the extended layer, which is a less-reliable channel, is divided into a space layer for increasing accuracy of an image, a time layer for increasing frame rates of a moving image, and a signal to noise (S/N) ratio layer for increasing S/N ratios of an image.
The extended layer may not be processed using redundancy checks such as FEC. Further, when the circuit status becomes undesirable, the extended layer may not be transmitted to the reception device. Because the essential base layer is always transmitted to the reception device, the extended layer that is transmitted from the transmission device without errors and the base layer may be combined and decoded, to obtain a transmitted image result that is suitable for the circuit status.
However, according to Japanese Patent No. 3757857, when image data coded using the SVC method (hereinafter, referred to as “SVC image data”) is transmitted, a real time protocol (RTP) is used as a transfer protocol, which does not retransmit data. Therefore, the SVC image data may be transmitted having more priority in real-time data transmission rather than priority in reliability of data transmission.
Also, a data transmission system according to Japanese Patent No. 3757857 uses RTP, which does not retransmit data in principle, as a transfer protocol, but rather retransmits a portion of layers.
However, even when streaming moving images and the like, a transmission control protocol (TCP), which maintains reliable transmission by retransmitting data, may be used as a transfer protocol. When transmitting SVC image data through TCP, if a transmitted image packet that failed to be transmitted to the reception device exists, then, all of the non-transmitted image packets are retransmitted. Thus, when transmitting SVC image data through TCP, even if the purpose of data transmission is only to reproduce moving images, transmitted image packets of the extended layers that do not necessarily have to be transmitted to the reception device are still completely retransmitted. Therefore, even though picture quality of moving images may be fixed at best quality, the real-time feature is degraded because of the delay caused during the receiving of every transmitted image packet.
That is, simply by combining TCP and the SVC method, various picture qualities that depend on the circuit status may not be automatically obtained, and the same best-effort image result as using a combination of RTP and the SVC method may not be obtained. Accordingly, because features of the SVC method may not be implemented, data may be encoded in various bit rates depending on the circuit status as in other related technology, or a plurality of encoded moving image data may be provided.
Also, in a data transmission system according to Japanese Patent No. 9-307510, assuming that a transmission device repeatedly retransmits image packets until the transmitted image packets are received as reception completed packets at a reception device, the data transmission system includes features that only use the reception device to ensure that a base layer is received in the reception device, and at the same time, does not retransmit image packets for other non-received extended layers.
Specifically, the reception device may not transmit the reception completed packet for a non-received base layer to the transmission device. At the same time, even when extended layers are not actually received, the reception completed packets for non-received extended layers may be transmitted to the transmission device, causing a forced stop of the retransmission process regarding the non-received extended layer.
Further, in such a reception device, if there is even one transmitted image packet of the extended layers that could not be received in the reception device, then, the retransmission of the extended layers is stopped. Therefore, if the circuit status is undesirable, the extended layers are almost not decoded at all, and only the base layer is transmitted.
In other words, regardless of the circuit status, even if there is a need to provide a minimum protection of the base layer and first-priority extended layer to reproduce a moving image, because the first-priority extended layer cannot be protected, the transmission device may not be able to provide best-effort image data transmission that reflects the user preferences regarding picture quality.